Add a way to uninstrument JVM metrics

[timmc]

If I call metrics.jvm.core/instrument-jvm during startup, there is no corresponding metrics.jvm.core/uninstrument-jvm to call during shutdown. This causes problems for me during tests that check startup and shutdown behavior; if I call instrument-jvm twice I get an error "A metric named jvm.attribute.vendor already exists".

I suppose a less desirable alternative that would still address my use-case would be to make instrumentation idempotent. A clean shutdown would be my preference.

[timmc]

Implementation note: remove-metric is not the inverse of add-metric since the latter will expand MetricSets into multiple names and metrics. :-(

dropwizard/metrics#1007